The field of application of the invention lies in particular in manufacturing composite material parts comprising a porous substrate or "preform" densified by a matrix.
To manufacture composite material parts, in particular thermostructural composite material parts constituted by a refractory fiber preform (e.g. carbon or ceramic fibers) densified by a refractory matrix (e.g. carbon or ceramic), it is common practice to use chemical vapor infiltration methods. Examples of such parts are carbon--carbon (C--C) composite nozzles for thrusters, or C--C composite brake disks, in particular for airplane brakes.
Densifying porous substrates by chemical vapor infiltration consists in placing the substrates in a reaction chamber of an infiltration installation by means of support tooling, and in admitting into the chamber a gas having one or more components constituted by precursors for the material that is to be deposited within the substrates for the purpose of densifying them. Infiltration conditions, in particular gas composition and flow rate, and also temperature and pressure inside the chamber are selected to enable the gas to diffuse within the accessible internal pores of the substrates so that the desired material is deposited therein by a component of the gas decomposing or by a reaction between a plurality of the components thereof.
The conditions required for chemical vapor infiltration of pyrolytic carbon or "pyrocarbon" have been known for a long time to the person skilled in the art. The precursor for carbon is an alkane, an alkyl, or an alkene, generally propane, methane, or a mixture thereof. Infiltration is performed at a temperature of about 1000.degree. C. at a pressure of about 1 kPa, for example. The infiltration conditions required for chemical vapor infiltration of materials other than carbon, in particular ceramic materials, are also well known. On this topic, reference may be made in particular to document FR-A-2 401 888.
In an industrial installation for chemical vapor infiltration, it is usual to load the reaction chamber with a plurality of substrates or preforms to be densified simultaneously, by using support tooling comprising, in particular, trays and spacers. When the preforms are annular, they may be stacked in a longitudinal direction of the reaction chamber. The gas containing the precursor(s) of the material to be deposited within the preforms is admitted at one longitudinal end of the chamber, while the residual gas is evacuated from the opposite end where it is extracted by pumping means. Means are generally provided to preheat the gas before it reaches the preforms to be densified, e.g. means in the form of perforated preheating plates through which the gas passes on being admitted into the reaction chamber.
A real difficulty encountered with known chemical vapor infiltration methods is to ensure that the microstructure of the material deposited within the substrates is constant. In the particular case of composite material parts, the expected properties of said parts require the microstructure of the matrix to be constant and of the kind desired. Thus, in the example of infiltrating pyrolytic carbon or "pyrocarbon", variations in infiltration conditions, even very small variations, can lead to changes in the microstructure of the pyrocarbon. Unfortunately pyrocarbons of the smooth laminar type, of the rough laminar type, and of the isotropic type have properties that are quite distinct. For example, if it is desired to obtain a graphitable pyrocarbon matrix by heat treatment, it is preferable to obtain a rough laminar type microstructure. In practice, in spite of the care given to controlling infiltration conditions, changes are observed in the microstructure of the pyrocarbon deposited within preforms, in particular within the preforms that are furthest from the access for the gas into the chamber. Such irregular microstructure has sometimes gone as far as forming soot and as forming undesirable dendritic growths in the reaction chamber.
To solve that problem, attempts have been made to significantly increase the flow rate of the gas admitted into the chamber, such that similar gas is presented to all of the preforms in the load. However it is then necessary to provide a more powerful pumping device, which is therefore more expensive, and more gas is consumed. In addition, the effectiveness of the preheating is decreased if the gas passes more quickly through the preheater plates. To bring the gas to the desired temperature not later than its first contact with a preform to be densified, it is necessary to increase the number of preheating plates, but that is detrimental to the working volume available inside the chamber, and thus to the overall throughput of the installation.